a) Tell them you'll consider.. if they ask "Huh? You come and apply for our company and now you are considering?", then tell them the truth, you have another interview and the reason why you are here is because are are keen to join their company but you are there to find out how are things around there AND you would also like to consider the other company..

b) You accept another job did you sign the letter of offer yet? if you have, its not as easy to just say okay i dowan work for u guys. If its just word of mouth then you can always let them know u got an offer elsewhere which you feel more comfortable with.